As you can see from the photo, we went well past the recommended 3 month usage for our HVAC's air filters. There is something horrifying and yet strangely fascinating about the amount of, well, crap, that can build up in an air filter. We'd rather not think about what we've been breathing in all summer.
It was so simple to change, we feel silly for not doing it sooner. They pop right in and out of our vents and took us all of 5 minutes to do. New filters are available at most hardware stores in a variety of sizes and brands. We've even seen them at Target. Prices can range dramatically depending on the brand and the duration of use (30 days to 3 months).